Code division multiple access (CDMA) is a popular multiple access technique that is used to support multiple users simultaneously in a network. Many variants of CDMA exist, including direct sequence (DS) CDMA, multi-carrier (MC) CDMA, cyclic prefixed (CP) CDMA, and chip interleaved block spread (CIBS) CDMA. In addition to these variations, many receiver architectures are often available for implementation in CDMA systems, such as the well-known RAKE receiver, interference cancellation receivers, and receivers that rely on channel equalisation.
Some CDMA schemes are interference limited; that is to say, as the number of users in the network increases, residual interference caused by each user eventually cripples the network, thus rendering simultaneous multiple accesses nearly impossible. This residual interference generally results from the loss of orthogonality amongst users, which primarily occurs when the channel is temporally dispersive. Several recent developments in block CDMA systems, including so-called ‘generalised MC-CDMA’ (GMC-CDMA), CIBS-CDMA, single-carrier frequency division multiple access (SC-FDMA; also known as DFT-spread OFDM), a throughput-efficient block CDMA system and configurable schemes have led to multi-user interference (MUI) free transmission techniques.
US 2004120274, incorporated herein by reference, discloses CDMA transceiver techniques for wireless communications.
US 2002126740, also incorporated herein by reference, proposes chip-interleaved, block-spread multi-user communication.
An example of a so-called ‘generalised MC-CDMA’ (GMC-CDMA) system is described in “Wireless multicarrier communications”, Zhendao Wang, Giannakis, G. B., Signal Processing Magazine, Vol. 17, No. 3, pages 29-48, May 2000, incorporated herein by reference.
Shengli Zhou, Giannakis, G. B., Le Martret, C. describe a system using CIBS-CDMA in: “Chip-interleaved block-spread code division multiple access”, IEEE Transactions on Communications, Vol. 50, No. 2, pages 235-248, February 2002, incorporated herein by reference.
Single-carrier frequency division multiple access using DFT-spread OFDM is described in “Performance comparison of distributed FDMA and localised FDMA with frequency hopping for EUTRA uplink,” NEC Group and NTT DoCoMo, TSG RAN WG1 Meeting 42 R1-050791, August 2005, incorporated herein by reference, and “A low complexity transmitter structure for OFDM-FDMA uplink systems”, D. Galda and H. Rohling, in Proceedings of the IEEE Vehicular Technology Conference (VTC), Vol. 4, pages 1737-1741, May 2002, also incorporated herein by reference.
A throughput-efficient block CDMA system has been proposed by S. Tomasin and F. Tosato: “Throughput Efficient Block-Spreading CDMA: Sequence Design and Performance Comparison” in: Proceedings of the IEEE Global Telecommunications Conference (Globecom), November-December 2005, incorporated herein by reference.
In MUI free systems, any number of users—up to a given maximum number—can theoretically transmit simultaneously without causing any degradation in system performance. Beyond this maximum number of allowable users, the system becomes interference limited in a similar manner to other CDMA systems.
Configurable CDMA schemes are disclosed in GB 2 433 397 A and in J. P. Coon; “Precoded Block-Spread CDMA with Maximum User Support and Frequency-Domain Equalization,” in Proc. of the IEEE International Conference on Communications (ICC), Glasgow, 2007. These techniques have lead to mulit-user interference (MUI) free transmission techniques. In these systems, any number of users—up to a given maximum number—can theoretically transmit simultaneously without causing any degradation in system performance. Beyond this maximum number of allowable users, the system becomes interference limited in a similar manner to other CDMA systems.
Although many prior art systems can provide MUI free and inter-block interference (IBI) free transmission using the block CDMA techniques, this can generally only be achieved when perfect synchronization of the users can be obtained at the receiver. In practice, signal reception among different users cannot usually be perfectly synchronized due to delays in transmission or the delays from the channel. As a result, residual MUI and IBI will occur. In “A simplified transceiver structure for cyclic extended CDMA system with frequency domain equalization” (Xiaoming Peng, Francois Chin, T. T. Tjhung and A. S. Madhukumar, Proc. Of the Vehicular Technology Conference, 2005, VTC 2005-Spring, 2005 IEEE 61st, June 2005, p. 1753-1757) the authors propose a method based on the use of a cyclic prefix and a cyclic postfix (or suffix) that can be used in BS-CDMA systems to mitigate the effects of asynchronous reception. This approach, however, requires the additional overhead of long, and frequently spaced, cyclic extensions, thus limiting the bandwidth of the system.